This invention relates to a star polymer reactive colloidal composition, and to one part coating compositions containing the colloidal composition. The star polymer has latent cross-linkable functionality. Coating compositions containing the colloidal composition have improved mechanical properties, film strength, block resistance, wet adhesion, and abrasion resistance.
Surface active agents, or surfactants are used to provide stability for emulsion polymer particles. These emulsions can then be used in the production of emulsion coatings. A problem with coatings produced with surfactants is that over time, the surfactants can migrate to the surface of the coating, producing detrimental effects on the surface properties. These detrimental effects are especially negative in mechanical properties related to hardness.
U.S. Pat. No. 5,605,952 describes a coating composition comprising polymers having acetoxy functionality and polymers having acid functionality, which can form a stable enamine structure by reacting with an amine. These polymers, however, produce a linear polymeric structure. An advantage of star polymers is that they have a lower viscosity than linear polymers.
U.S. Pat. No. 5,274,064 describes star polymers with reactive functional groups. The star polymers disclosed are backbone polymers to which alkoxy Si groups are attached as side chains. Such side chains are not highly sterically hindered, and therefore can react prematurely to form crosslinks prior to film formation, causing coagulum problems during manufacture and storage, or producing surface crosslinking between emulsion particles, rather than stronger crosslinking formed after particle coalescence. The radial star polymers described are the sole constituent of a coating composition, rather than an emulsifier useful in producing a coating composition.
U.S. patent application Ser. No. 09/227,756 describes coating compositions prepared with a sterically hindered alkoxylated silane. These coating compositions are formed from linear polymers, and lack a controlled architecture.
U.S. patent application Ser. No. 09/190,527 describes the use of amphiphilic heteroarm star polymers as emulsion stabilizers in emulsion polymerization.
A useful one-part reactive coating composition should be stable. It should not react during the polymerization process, during storage, nor during the liquid coating stage. Surprisingly, it has been found that star polymer colloids of the present invention, having potentially cross-linkable groups on the polymer backbone fulfill these requirements, providing excellent stability and also physical coating properties when used as stabilizers in emulsions used as part of a coating composition.
The present invention provides a colloid composition comprising an amphiphilic star polymer having potentially crosslinkable sites on the polymer backbone consisting of either a sterically hindered silane monomer and an internal catalyst, or an acetoacetoxy group and at least one non-polymeric polyfunctional amine.
The present invention also provides a means of producing an emulsion polymer using the novel colloidal composition of the invention as a replacement for surfactants for stabilization.
Further, the present invention provides a one-part coating composition comprising a radial or star polymer having cross-linkable functionality and an emulsion polymer.
While not being bound by any particular theory, it is believed that the colloids of the present invention react primarily after coalescence has occurred, due to the high level of steric hinderance provided. Because the reactive groups on the star-polymer colloid are so hindered, a reaction occurs only after the reactive groups are forced into very close proximityxe2x80x94a condition which occurs after coalescence of the emulsion particles. This mechanism is different than the prior art having polymer reactive groups which form surface cross-links between adjacent polymer chains, providing a weak bond and weak coating film. Ideally the colloids of the present invention migrate into and intermingle with other polymer chains, prior to the formation of crosslinks. The result is a much stronger, more cohesive bond and water resistant film.
The protective colloid composition of the present invention is an amphiphilic star polymer containing potentially crosslinkable sites on the polymer backbone. As used herein the term amphiphilic star polymer refers to a polymer having both hydrophilic and hydrophobic components.
Star or radial polymers, as used herein, is intended to describe polymers that have three or more polymeric arms emanating from a central core. These polymers can be prepared by various polymerization procedures such as anionic, cationic, and free radical mechanisms. The star polymers are usually formed by using either multifunctional initiators, multifunctional chain transfer agents, or multifunctional coupling agents. The star polymers have unique properties including low viscosities in solution due to their compact structure, and high melt viscosities due to extensive entanglements relative to their linear coatings.
Preferably, star polymers of the present invention comprise a polyvalent mercaptan core and three or more polymeric arms which extend radially from the core.
Preferably the core is a residue of a tri- to octafunctional thiol, and most preferably a residue of a tri-, tetra-, or hexafunctional thiol. The arms of the radial or star polymer may be of several types, including random or block copolymers, or homopolymers. The arms may be of the same or different compositions. A preferred composition is one in which the all of the arms are essentially the same. Other preferred star polymers are those which are heteroarm star polymers.
Said heteroarm star polymer has at least one arm with a Tg of at least 20xc2x0 C. for a stable emulsion, preferably at least 25xc2x0 C. and most preferably at least 50xc2x0 C. If all arms of the star polymer are essentially the same, the Tg of the arms is at least 20xc2x0 C. or greater.
Preferably, the arms of the star polymer contain 5-20% by weight of an anionic monomer such as methacrylic acid and 1 to 50% by weight of a cross linking functionality, based on the star polymer.
The star polymer useful in the present invention has a number average molecular weight of from 10,000 to 100,000; preferably 15,000 to 75,000; more preferably 30,000 to 75,000; and most preferably 30,000 to 50,000, based on a theoretical molecular weight calculated as
Mw=(grams of monomer/moles of chain transfer agent)*n,
where n equals the number of arm on the star polymer.
Potentially crosslinkable sites, as understood herein, relates to functional groups which can react to form crosslinks, primarily during film formation after the onset of coalescence. Examples of potentially crosslinkable systems useful in the invention include, but are not limited to, chemical crosslinking, ionic crosslinking, and oxidative crosslinking.
Chemical crosslinking of the amphiphilic star polymer can result from several co-reactive groups, including but not limited to, sterically hindered silane, and acetoacetoxy functional chemistries. Other means of crosslinking include ionic crosslinking, such as a post-addition of a zinc ammonium complex to the latex; and oxidative crosslinking, as with dicyclopentenyl acrylate and castor acrylated monomer.
A star polymer having sterically hindered silane, is the reaction product of a residue of a tri-to-octafunctional thiol, an ethylenically unsaturated non-carboxy functional monomer, from 1 to 20 and preferably 2 to 10 parts per hundred monomer (pphm) of a sterically hindered alkoxylated silane monomer, optionally from 0.1 to 5 pphm of an ethylenically unsaturated carboxy-functional monomer, optionally from 0.1 to 5 pphm of a wet adhesion promoter such as an ureido-functional monomer, and an anionic surfactant. The sterically hindered alkoxylated silane is incorporated in the backbone of the polymer. The steric hindrance of the alkoxylated silane minimizes hydrolysis of the alkoxylated group during polymerization and storage.
While not wishing to be bound by any particular theory, the present inventors believe that crosslinking occurs between the sterically hindered alkoxysilane functionality on the polymer by means of a hydrolysis reaction to give silanols with subsequent condensation reaction between silanols and/or carboxyl groups on the polymer. Such crosslinking occurs during film formation, most probably after particle coalescence or drying of the coating. The advantage of preparing the coating composition with sterically hindered alkoxylated silane monomers is that crosslinking during the emulsion polymerization of the polymer and storage of the coating, especially in the presence of carboxyl groups, is minimized.
The sterically hindered alkoxylated silane monomer has the structure:
R1xe2x80x94Sixe2x80x94(OR2)n(R3)3xe2x88x92n
wherein R1 is selected from the group consisting of an alkylene, arylene, and aralkylene group; R2 is independently a sterically hindered alkyl group having 3 to 10 carbon atoms in a straight or branched chain configuration; R3 is a monovalent alkyl group having 1 to 10 carbon atoms; and n is an integer of from 1 to 3.
Suitable sterically hindered alkoxytated sllane monomers for use In the ooating compositions of the invention are vinyltrisopropoxy silane, vinyltriisobutoxy silane, vlnylpropylmethyipentoxy silane, vinylpropyidi-sec-butoxysilane. The sterically hindered alkoxylated silane monomer is preferably vinyltriisopropoxy silane.
Commonly used silanes, such as vinyl trimethoxysilane, vinyl trimethoxy silane, or methoxy diethoxy vinyl silane would not constitute a sterically hindered silane, and would react too quickly, creating premature crosslinks.
Star polymer colloids having a sterically hindered silane require an internal catalyst to provide crosslinking. An internal catalyst is one which is part of the polymer backbone, and serves as a catalyst in the formation of crosslinks with the silane functionality. An example of an internal catalyst would include, but not be limited to, an ethylenically unsaturated carboxy-functional monomer.
The internal catalyst is used in an amount of from about 0.1 to about 10 pphm, preferably from about 0.5 to about 2 pphm. An external catalyst, such as zinc or aluminum ions could additionally be present.
Suitable ethylenically unsaturated carboxy-functional monomers are xcex1, xcex2-ethylenically unsaturated C3-C8 monocarboxylic acids, xcex1, xcex2-ethylenically unsaturated C4-C8 dicarboxylic acids, including the anhydrides thereof, and the C4-C8 alkyl half esters of the xcex1, xcex2-ethylenically unsaturated C4-C8 dicarboxylic acids. Preferred ethylenically unsaturated carboxy-functional monomers are acrylic acid, methacrylic acid, and the C4-C8 alkyl half esters of maleic acid, maleic anhydride, fumaric acid, carboxyethylacrylate, and itaconic acid. Most preferably, the ethylenically unsaturated carboxy-functional monomer is acrylic acid or methacrylic acid. A combination of ethylenically unsaturated carboxy-functional monomers may also be used to prepare the star polymer.
Acetoacetoxy functional chemistries can also be used on the star-polymer backbone as the potentially crosslinkable sites. Acetoacetoxy functional chemistries useful in the present invention are monomers having the ability to form stable enamine structures by reacting with amines. The acetoacetoxy functionality can be added to the star polymer backbone by ethylenically-unsaturated acetoacetoxy monomers including, but not limited to acetoacetoxyethyl methacrylate (AAEM), acetoacetoxy ethyl acrylate, acrylamidomethylacetylacetone acetoacetoxybutyl methacrylate, allyl acetoacetate, vinyl acetoacetate and combinations thereof.
Crosslinking of a star polymer containing an acetoxy-functioanl moiety such as AAEM or enamine requires reaction with a non-polymeric polyfunctional amine. The polyfunctional amine is a separate post-add to the colloid composition, or to an emulsion polymer containing said colloid composition. A polyfunctional amine is one containing at least two amine-functional moieties. Polyfunctional amines include, but are not limited to, aliphatic and cycloaliphatic amines having 2 to 10 primary and/or secondary amino groups and 2 to 100 carbon atoms. Examples of useful polyfunctional amines include hexamethylene diamine, 2-methyl pentamethylene diamine, 1,3-diamino pentane, dodecane diamine, 1,2-diamino cyclohexane, 1,4-diamino cyclohexane, para-phenylene dianine, 3-methyl piperidene, isophorone diamine, bis-hexamethylene triamine, diethylene triamine, polyfunctional amines containing adducts of ethylene and propylene oxide, such as the JEFFAMINE products of Huntsman Chemical Company.
The star polymer is made by free radical solution polymerization, followed by a solvent replacement to form a colloid composition. This colloidal composition can then be used as a stabilizer in the preparation of emulsion polymers.
The colloidal composition of the present invention can be used in place of surfactants typically used to manufacture emulsions. The present invention also provides a means of producing an emulsion polymer using the novel colloidal composition of the invention as a replacement for surfactants. The colloid can be used in the same manner, and at the same concentrations typical of surfactant use. A preferred usage would be from 5 to 20 parts per hundred monomer. Emulsion polymers containing monomers having any reactive moiety are especially preferred, since these provide potential sites for reaction with the colloidal composition, though a monomer containing a reactive group is not needed, since the reactive star polymer colloid can intertwine and tangle any polymer chains prior to crosslinlking with itself or onto another colloid particle, creating entanglements which result in improved physical properties. Emulsion polymers containing acrylic and vinyl monomers are especially preferred. A wet adhesion monomer may optionally be present in either the star polymer or emulsion polymer.
Although the solids content and viscosity of the emulsion can vary, typical total solids content which is defined as the nonvolatile components of the emulsion is preferably in the range of from about 40 to about 70 weight percent, more preferably from about 50 to about 60 weight percent, based on the total weight of the emulsion.
Coating compositions of the present invention are those containing an emulsion polymer made with the colloid composition described herein. The one-part coating compositions of the present invention are prepared as aqueous compositions which are curable to form a film. Preferably the coating composition is free of surfactants, using only the colloid composition of the invention to stabilize the emulsion.
The coating composition may additionally contain other additives which include pigments such as titanium oxide, extenders such as flour, i.e., walnut shell flour, dispersing agents, defoaming agents, anti-freezing agents, preservatives, surfactants, sequestering agents, coalescing agents, defoaming agents, humectants, thickeners, defoamers, colorants, waxes, bactericides, fungicides, and fillers such as cellulose or glass fibers, clay, kaolin, talc, calcium carbonate and wood meal, and odor-modifying agents.
In preparing the coating composition of this invention, the emulsion polymer is mixed with the additive(s). The additive(s) may be added during the polymerization, or after the polymerization. Coatings produced in this manner include high-gloss, semi-gloss and low odor paints, and pressure sensitive adhesives.
The coating composition may be applied to a wide variety of materials such as, for example, wood, cement, concrete, leather, nonwoven or woven fabrics, aluminum or other metals, glass, ceramics, glazed or unglazed, tiles, polyvinyl chloride and polyethylene terephthalate and other plastics, plaster, stucco, roofing substrates such as asphaltic coatings, roofing felts, synthetic polymer membranes, and foamed polyurethane insulation. In addition, the coating compositions may be applied to previously painted, primed, undercoated, wom, or weathered substrates.
The following nonlimiting examples illustrate further aspects of the invention.